Author Topic: P&W Wasp Jr. in 1/6th scale  (Read 7153 times)

Offline Vixen

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #15 on: March 19, 2020, 12:52:11 PM »
Hello Mike R

I agree with Jason about the use of DCGT inserts and making double ended pairs of valves. I go as far as also turning the outside diameter of the bar stock after turning the first valve. This to ensure the stock is round and concentric with the first valve, before it's reversed for the second valve.

I also have a EMCO Compac 5 CNC which I use for this sort of work. It is a light, flexile (as in bendy) machine but with light cuts and the right tooling, it can do great things. I discarded my Emco auto tool changer in favour of a quick change tool holder and half a dozen tool holders. This allows you to get the saddle closer to the work and reduces tool push off. Unfortunately, this makes Jason's suggestion of a rotary centre, more difficult.

For any work that may require accurate repositioning, I use the EMCO collet adapter. I know it's an ugly device with about the same overhang as the three jaw, but it is so much more accurate at recentering than the three jaw. A set of metric collets will cover the whole range of diameters from 16mm down to 1mm. You dont need to buy 'special' imperial collets, you can hold anything round in one of the metric collest.

Mike
« Last Edit: March 19, 2020, 01:01:04 PM by Vixen »
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Offline Mike R

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #16 on: December 21, 2020, 02:26:50 AM »

Well, I haven't revisited the valves with any kind of commitment yet, though I did happen to cut a piece of 1144 steel (3/8" diameter) on my (big) lathe to make a pin, turned it down in 1 step and boy oh boy did that cut nice.  I may give it a go instead of the 303 SS.


What have I accomplished?  A whole lot of non-model engine stuff (rebuilding a cottage).
And a lot of shop related stuff (rewiring the mill was a big one, lathe metric gear change set, etc.)


On the engine:
Looking over things, its has been a long time since I posted. 


I had been playing with gears for the lathe and gave making the gears for the engine a go.  I decided to redo the layout of the geartrain to use metric gears (mod 0.5) versus the as drawn DP48, as the cutters were significantly less expensive. 


So after drawing it up in CAD to be sure it was all going to work I set about making some gears.
They turned out to be relatively easy to make with the 4th axis setup and turned out quite nice.


I just recently got the master connecting rod fabricated, after much relearning about my mill and its capabilities and more importantly, limitations.


Not perfect but as it will live hidden inside I may just leave it - some small cosmetic issues to do with the head on the mill rocking around under high cutting forces or even just high accelerations.


I may have lost count of the number of small tools I broke getting the final product, but at least being small they cost less to learn with.


Also half fabbed up the 8 pins to hold the link rods.  The drawing calls for some special geometry on the ends that I'll need to setup to do (small step at half width on either end).


I also started on the crankshaft but stopped while I got the mill rewired.  So lots of tasks to pick up again (valves, crankshaft), but am hopeful that as I spend more time in the shop I'll spend less time making scrap and more time on useable bits.
« Last Edit: December 21, 2020, 02:30:45 AM by Mike R »

Offline Mike R

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #17 on: January 05, 2021, 11:16:19 PM »

So I had another go at making the valves -  10 months later!


Taking advice given I made the following changes:


I added an ER32 collet chuck to the lathe
I used a DCGT type insert (i.e. sharp insert intended for aluminum)
I set the insert holder at an angle to the axis to allow clearance while profiling
I skipped the parting off and used pieces cut to 2 x valve length + about 20 to 25mm


Another fairly major change I made was to increase the speed of the spindle. I had been running the motor flat out, but was in the wrong belt position and had a limited speed at the chuck (probably ~1000rpm).  With the belt moved over I found a happy speed at ~2300rpm (higher and there is some awful lathe/motor noise produced even without cutting!).


I programmed in some backlash compensation on the cross slide (X axis) which helped with shaping the head area.


And lastly I changed over to 1144 steel - this last one was more from being out of stock of the 304 stainless I had been using.


To show the improvement I have taken a picture of a previously made valve with a new one.
You can clearly see the surface finish improvement.
Also, most have come out within a half thou of the desired size (0.0937")  Chasing that last one, to half-thou variation is a real pain, and I'm not confident I'll solve it with a reasonable amount of effort.
For now I'm happy as I can likely adjust the valve stem diameter down 0.0005 to 0.001" by lapping if needed -I'll wait until I've made the valve cages to see how close a fit I can achieve.

Offline cnr6400

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #18 on: January 05, 2021, 11:49:40 PM »
 :ThumbsUp: :ThumbsUp: :ThumbsUp: :popcorn: :popcorn: :popcorn:

Outstanding improvement on the old vs new valves! Hope you collected arr that swarf - you could start a pot scrubber manufacturing business!  :Lol:   :cheers:
"I've cut that stock three times, and it's still too short!"

Offline Vixen

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #19 on: January 06, 2021, 06:05:22 AM »
Pleased the advise proved useful. Nice finish on those valves :ThumbsUp: :ThumbsUp: :ThumbsUp:

Mike
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline Mike R

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #20 on: January 09, 2021, 12:34:34 AM »

So I thought I'd keep at the valve train and see if I could do some work on the rockers.  Need to make 9 each of a left and right (intake and exhaust).  As drawn there are 2 options, one carved from solid phos. bronze, and 1 from 7075 aluminum with an oilite bushing pressed in.  The later is the one I had chosen to draw up. Regardless of the rocker design, the adjustable tappet screw at the end is the same.


The debate I'm having right now is the tappet screw end seems overly complicated and I'm looking for alternate designs before I begin making the rocker arm in case it is affected.  I don't intend to run this engine hard or long so longevity isn't a primary concern, though I do not want it to chew itself to pieces after 20 minutes.


Attached is section view of the rocker arm and tappet screw. The screw is simply called an M4 on the drawing (presumably M4 x 0.7 but could be x 0.5).


Inserted in the end of the short screw is a 3/32" ball that is captive and has a flat ground on it, but is intended to be free to rotate in the screw thereby always pressing with the flat on the valve stem.
The screw also has a 30deg bevel at the ball end.


Seems like those are going to be a challenge to make.  Anybody have options on simplifying this setup, or am I worried about nothing? 
Unfortunately,  I can't move the adjustment feature to the pushrod end due to the head casting being closed over back there (see picture).


Regards,


Mike




Offline cnr6400

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #21 on: January 09, 2021, 01:50:52 AM »
Hi Mike. the ball with the flat seems like a difficult way to get a hardened surface running on the valve stems. The other thing is, unless the setscrew is long enough to accept a nut, or the setscrew is pinch clamped somehow, the setscrew will likely start rotating and loosen itself, ruining your hours of valve adjustment quality time!

Maybe a longer setscrew (no inset ball) made of 0-1 tool steel and through hardened, then tempered to straw colour, would be better. A "model size" M4 nut, with reduced hex size vs screw dia, could be used to allow locking the setscrew in place on the rocker after adjustment. The smaller hex would give a more "scale" appearance, although a normal M4 nut would likely  function just fine. I'd also suggest at least 4 full threads in the rocker, or more, even if you have to thicken it a bit on the outside face to get the additional threads. This will help stabilize the setscrew and reduce risk of it and the nuts coming loose. Just food for thought.  :cheers:
"I've cut that stock three times, and it's still too short!"

Offline Mike R

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #22 on: January 09, 2021, 04:03:20 PM »
Thanks CNR,
The rocker arm does call for it to be split at the end and pinched to retain the tappet screw.
I'm also not opposed to a little chemical fixation to keep things in place once I'm happy (loctite)
Digging through the drawings much further towards the back of the set I came across a valve train layout drawing that helps me understand better the space constraints, while keeping things looking scale from the outside.
There's no room for a protruding screw and extra nut as it would interfere with the valve cover, and even as drawn this design is very close with no extra space.
The only other idea I've had is to make the rocker arm fixed with a curved tappet face similar to most commercial small 4 strokes (creates a line contact on the valve stem face) and then to adjust I'd need to
  • do something with the pushrods (again not possible due to pushrod tubes covering the pushrods)
  • make the rocker pivot have an eccentric relative to its mounting hole in the head. Figuring out the right amount of eccentricity to provide adjustment that is reasonable is a big experiment I don't think I want to take on.
I may just soldier on and make as drawn.  Hopefully it doesn't drive me crazy :insane:

Offline cnr6400

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #23 on: January 09, 2021, 09:32:29 PM »
Hi Mike, Looking at the valve rocker housing design section drawing you posted, it sure isn't optimal for space inside! I think the ball in screw design is a pretty bad / optimistic design feature by the designer, at the M4 screw size noted. If it was a 10 mm dia screw, and the socket could be machined with a ballnose cutter with lots of metal either side, different story maybe......

Apart from the adjustment need, a cylindrical roller mounted on a pin in the rocket might be better in terms of lube and reduced friction vs the ball setup. If you do change plan to adjustment by eccentric pivot bolts for the rockers, the roller might be worth considering. The rocker would have a slot cut in it to receive the roller. if a case hardened dowel pin is used with a cast iron or bronze roller, parts will run nicely together and there is a flow thru path for lube oil. The rocker holes for the dowel could be peened at both sides to keep the pin retained. Very space efficient.  Just a thought. Roller rockers are very common and well proven in high performance V8 car engines.

In any case, good luck with it.  :cheers:
"I've cut that stock three times, and it's still too short!"

Offline dieselpilot

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #24 on: January 09, 2021, 10:00:25 PM »
I would think that was intended to be modified from a flat ball point set screw. They didn't make this clear in the prints/parts list?

https://www.elesa-ganter.com/en/www/Machine-elements--Ball-point-screws--GN605

Offline petertha

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #25 on: January 09, 2021, 10:43:16 PM »
{deleted post, I misunderstood sketch}



Offline Mike R

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #26 on: January 10, 2021, 03:44:25 AM »
Dieselpilot - thanks for the link - buying this premade is a real good idea.  At worst I just have to modify it slightly. 

B.T.W. the instructions are:
"Tappet Screw - Cut from M4 M.S. screw.  Lightly crimp to retain ball. Ball must swivel freely"
And the socket for the ball is indicated to be made with 3/32 bull nose endmill (my belief is that is meant to be ball nose but who knows...)
And the ball is indicated to have 0.4mm taken off to create the flat.

CNR - A roller tip is a good idea if needed to go a different route than as designed.  But still would need to figure out an adjustment mechanism that is reasonable to build in this scale.  I also realized that making the rocker shaft eccentric wouldn't work unless I opened up the shaft holes through the head and then put bushings,  starts to get ugly quick.

I think I'll try building it as designed, with purchasing the premade ball tip setscrews, and hope it works well enough for the little running it will get

Offline Mike R

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #27 on: January 26, 2021, 02:30:17 AM »
Pistons (and rings)

As I wait for various materials and items to show up I thought I could tackle the machining of the pistons. 
The pistons are supplied as a casting but they leave very little material for clean up.  I also only have 9 so no practice parts allowed!
The desired finished piston diameter is by the drawing is 0.002” under the bore, so 0.873”
The as cast diameters vary from max ~0.899 to min 0.884” meaning at best case I have 0.013” to remove (radius) and worst case only 0.0055”.
The drawing set does call up a method of machining that I have decided to use as it seems likely to succeed. 
The piston is chucked up in a collet and  the skirt lightly is lightly faced, and a small counter bore machined 1/16” deep and to 0.740” diameter (to sit on a fixture).

I machined up the fixture to hold the piston, centering the skirt on a 0.740” diameter shoulder protruding 1/16”.  The piston is then held by a pin through the wrist pin hole, into a Ό-20 screw with a loose fitting cross hole.
The pistons have cast into them the wrist pin hole and it was quite clean, allowing me to insert the reamer (7/32”) in via the shank end and to then clamp up the reamer in the drill and ream while pulling through, the shank acting as a guide to get the reamer started at the right angle.
And that is where I got to so far.

Reading ahead, I need to machine the diameter to size, and put in 2 piston ring grooves and this is the point where I again question the design.
1.   Are 2 piston rings really necessary on a cylinder with 7/8” (22mm) bore and 0.984” (24mm) stroke?  My thoughts are 1 will suffice but I have more research to do here first
2.   The piston rings are only 0.41mm thick (0.016”) and this may be the biggest reason for me to deviate from the drawings,  making rings (and good grooves) at this thickness will likely be my demise – I can envision broken rings littering my workshop, along with hundreds of pounds of cast iron swarf trying to generate 18 rings of acceptable fit and finish.
I have read up on making rings (https://www.modelenginemaker.com/index.php/topic,3794.msg68914.html#msg68914) but the decision as to how many is still up for debate. Following the guidelines in the referenced post, I see that my suspicions on ring thickness (w) are likely well founded from the following quote:
 Many published designs use rings which are too wide, & I suggest w = 0.03 x B as a guide, with a minimum of 0.04”.
Using the formula 0.03 x B (assuming this is Bore as there is no B elsewhere in the article) we get 0.03 x 0.875 = 0.0262” so I would be looking at the recommended minimum of 0.040” or if I’m daring the calculated 0.026” which is still 0.010" thicker than the drawings.  The radial width (t) would nee to be modified appropriately as well, again the drawings have it at a quite wide 0.065" versus the articles recommended D/25 to D/30 which works out to roughly 0.029" to 0.035"
At the end of the day, I’m looking to build a runner, not a power house.  So whatever in theory works best for an easy to start, lightly loaded and relatively low RPM engine in this size is what I’m aiming for. Being able to machine said parts is also a necessity as it will never be a runner if I can’t build it! I'm afraid I have more reading ahead before I continue with the piston machining.

Offline Mike R

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #28 on: March 07, 2023, 02:00:20 AM »
Time for some long overdue updates.  I'm going to start in with where I am as I am seeking guidance (see below).
I was performing some of the last major operations on the cylinder heads when disaster struck.
I've uncovered a casting flaw - a pocket of plaster / dust / sand whatever that was unseen previously but connects from the combustion chamber dome to the wall of the hole for the glow/spark plug.  This was found on the 8th of 9 castings (the 9th was ok).
The first picture shows it in the holding fixture when I realized that there wa a problem with the swarf coming out - it just wasn't right.
The next shows it after an inital blast with compressed air - I didn't realize just how bad this was yet as I've circled the barely visible material that I had not seen previously
The last picture shows that the defect connects the glow/spark plug hole to the combustion chamber after I blew it out with compressed air some more and picked at it with a scriber.



Now the question is - what to do about it.  I fear that the castings my not be available anymore though I am trying and track down if there's a successor making the castings that Bruce Satra made.
Failing to replace it I will need to repair it or face the reality of a non-operating engine (or cylinder). 


Repair thoughts?  Being in the hottest area of the engine of course, makes the idea of using an epoxy dubious at best.  Even the best epoxies are only rated to ~190C (400F) and I expect that local temperatures when running will exceed that.


TIG welding?  maybe if I pack the area with carefully collected shavings of the same casting material I can gently weld it being careful to not overheat the head.  being a casting I fear that it won;t take kindly to welding though and I've not got any spare material to practice with.


Other ideas?  the defect is at a strange angle and location so its not conducive to just drilling it out and plugging in a new piece and shaping it...


Mike




 

Offline vtsteam

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Re: P&W Wasp Jr. in 1/6th scale
« Reply #29 on: March 07, 2023, 02:06:29 AM »
What is the cylinder head material?
Steve

 

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